Low thermal conductivity cable core wrap

ABSTRACT

A thermally insulating laminate as a cable core wrap wherein the laminate consists of a at least one layer of a low bulk density, high-surface area material capable of intertrapping air in said laminate to lower thermal conductivity.

United States Patent Thomas J. Grail Plainiield, NJ.

Appl. No. 56,245

Filed July 17, 1970 Patented Oct. 19, 1971 Assignee Esso Research andEngineering Company Continuation of application Ser. No. 677,963, Oct.25, 1967, now abandoned.

lnventor LOW THERMAL CONDUCTIVITY CABLE CORE WRAP 2 Claims, 2 DrawingFigs.

[1.8. CI 174/107, 156/53, 156/56, 174/102 D, 174/1 10 PM, 174/121 Int.Cl 1101b 7/18, 1101b 7/34 Field of Search 174/107,

[56] References Cited UNITED STATES PATENTS 9/1967 Woodland et al 9/1967Kinney 4/1968 Bullock 10/1967 Blau et a] FOREIGN PATENTS 866,949 5/1961Great Britain Primary Examiner-E. A. Goldberg Assistant ExaminerA. t.Grimley Attorneys-Chasan and Sinnock and Michael N. Melier ABSTRACT: Athermally insulating laminate as a cable core wrap wherein the laminateconsists of a at least one layer of a low bulk density, high-surfacearea material capable of intertrapping air in said laminate to lowerthermal conductivity.

PATENTEDHCI 19 197i 0: AQHHHOV $5? QwEdd v mvzmoa PATENT ATTORNEY 1 LOWTHERMAL CONDUCTIVITY CABLE CORE WRAP This application is a continuationof Ser. No. 677,963 filed Oct. 25, 1967 now abandoned.

BACKGROUND OF THE INVENTION 1 Field of Invention This invention relatesto cable core wraps used as thermal insulation barriers to protect cablecores from heat damage during cable fabrication processes. Moreparticularly the cable core wrap consists of a laminated two orthree-ply material, one layer of which is of a very low thermalconductivity material.

2. Description of the Prior Art In the electric cable manufacturing art,multiple strands of wires are commonly protected by an aluminum sheath,preferably corrugated, in order to act as a conduit for the multiplestrands and also to protect said strands from physical damage. Thesecorrugated aluminum sheaths are then coated over by extrusion means withan appropriate plastic or rubber material so as to result in thecommonly used electric cable employed in the cable industry.

This extrusion process, however, produces considerable amounts of heat,so much so that some additional thermal insulation in necessary toprotect the strands of wires from melting their own individualinsulation and fusing together or has ing against said aluminum sheaths.This thermal insulation is commonly provided in the prior art asexemplified by US. Pat. Nos. 3,233,036 and 3,325,589 by a core tapewhich is a laminated film of GRS/Mylar or films of polyethyleneterephthalate, polypropylene, acetylated paper, etc.

While this thermal conductivity decrease goes far toward protecting thestrands of wires within the core, nevertheless increased thermalconductivity protection must be had in applications where the extrusiontemperature is higher for greater efficiency and where other reasons forgreater protection dictate a higher degree of thermal conductivityprotection.

SUMMARY OF THE INVENTION This need for a lower thermal conductivity corewrap has now been achieved by the use of a material having ahigh-surface area and low bulk density as compared with materials fromwhich it is formed (at most one-half the density of the originalmaterial), has also the further characteristic of low hydroscopicity andis suitable for short term use at high temperatures. Such low-densityhigh-surface area material is laminated to or sandwiched between a filmof continuous flexible dielectric material so as to intertrap air insuch laminate and to form a laminated core shield ready to be wrappedaround the strand of wires to be protected.

The purpose of this invention is, therefore, the protection of bulkmultiple strand cable cores by a low thermal conductivity laminate toprotect said cores from heat damage.

A further purpose of this invention is to provide an improved cableconstruction utilizing a laminated core wrap.

In accordance with these desired purposes, the laminated core wrap isconstructed of said low thermal conductivity material laminated to acontinuous flexible dielectric material, if two-ply or sandwichedbetween said continuous flexible dielectric material if three-ply.

The low thermal conductivity material is contemplated to have physicalstructure such as fabric (either woven or nonwoven), foam orcorrugation. The continuous flexible dielectric material laminatedthereto in a manner to give dead air spaces should have a dielectricconstant of 2.0 to 4.5, preferably not above 3.5 as measured at roomtemperature and at l kc., with a dissipation factor of 0.02 percent to1.5 percent preferably in the range of 0.04 percent to 1.5 percent.

Typical materials useful as either woven or nonwoven fabrics to be usedin the low thermal conductivity layer of the laminate include polyester(such as Dacron), acrylics (such as polyacrylonitrile), polyolefins(such as polyethylene and polypropylene), cellulose (such as celluloseacetate), and

glass fibers. Among the foam materials contemplated herewith are suchmaterials as foamed polyolefins, polyurethanes, etc. Among thecorrugated materials any of the categories of the materials falling inthe class of polyester films, polyolefin films, cellulosic films,polyphenyleneoxide films, polycarbonate films, etc., can be used.

In all of these applications it is necessary that dead air spaces beprovided for thermal insulation and that the highsurface arealow-density laminate be easily pliable so as to bend around the cablecore and impart a solid integrity to the strand of wires.

As the continuous flexible dielectric material, such materials aspolyolefins, including polyethylene and polypropylene; polyethyleneterephthalate, including MYLAR; NYLON; polyphenyleneoxide includingmodifications like NORYL and polycarbonates can be used in laminatedform with the low thermal conductivity material.

A cable constructed in accordance with this invention has a bundle orstrand of wires wrapped by the previously described laminate with thelow thermal conductivity material having a thickness of 2 to 25 mils,preferably 5 to 10 mils, while the dielectric material would be of theorder of from one-half mil to 15 mils, preferably 1 to 5 mils thick. Thecorrugated aluminum shield covering said core wrap would be of the orderof 8 to 20 mils thick, preferably 8 to 10 mils thick. The thickness ofthe polyolefin outside covering of the cable would generally be of theorder of 75 mils.

BRIEF DESCRIPTION OF THE DRAWING I I-Iaving briefly described theinvention, it will now be described in sufficient detail to show thepurposes and advantages of the invention and to clearly understand themanner in which the low thermal conductivity material is employed. Thiscan best be understood by reference to accompanying FIGS. 1 and 2.

In FIG. 1 there is a perspective view of a typical cable formingoperation, showing the actual plastic cable wrap being placed on thewires. FIG. 2 illustrates in cross section the finished cable asemployed.

In FIG. I there is shown a typical cable forming operation. The joinerll twists together the individual strands of wires 10. This strand isnow wrapped with the thermal shielding material 12 of this invention bymeans 13. This wrapped cable is now further shielded with slit aluminum14 which has been passed through corrugator l5 and passed through means16. The cable is now ready for passage through the crosshead extrusiondie 17 wherein it is coated with a layer of polyethylene. This coatedcable is quenched in tank 18, passed through continuous capacitancemonitor 19 and then taken to windup by means of the caterpiller haul-off20.

In FIG. 2 there is shown in cross section a telephone cable comprisingmultiple strands of insulated conductors assembled into a cable core 5about which is applied a thennal barrier 4 laminated to the dielectricmaterial 3. An aluminum sheath 2 is applied over said laminate withplastic jacket I extruded thereon.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In order to set forth in a morespecific fashion the details of this invention, the following examplessetting forth the twostep operation of laminate preparation and cablemanufacture for producing multistrand electric cables will be given asfollows:

EXAMPLE 1 length-to-diameter ratio. The extruder is fitted with a slotdie which enables a thin coating 56 inch wide to be placed on thenonwoven mat substrate. Extruding a polypropylene with a melt flowrateof 3 at 530 F. at a rate of 500 lbs/hr. and a coating line speed of 45feet/min. results in a 5 mil 5 polypropylene coating on the polyestermat. Positioning of the casting roll and extrusion nip in relation to thextrusion die is adjusted so that the hot extrudate contacts the cooledcasting roll before it is combined with the polyester mat, in order tominimize penetration of the polypropylene into the fibers. Thepolypropylene/polyester mat laminate is then trimmed in-line toeliminate the heavy coating edge and is wound in large diameter rolls(-28 inches). The rolls which are now approximately 54 inches wide areslit into narrow widths depending on the diameter of the cable core tobe wrapped. Here the final core wrap tapes are 6.75 inches wide.

Cable Manufacture-The cable to be made is an alpeth-type low voltagecommunication cable. The core is composed of 600 paired conductors,22-gauge copper, insulated with lowdensity polyethylene and bundled toan effective diameter of approximately 2.09 inches. The cable core isthen wrapped longitudinally with the previously made rolls of 6.75 inchwide core wrap, polyester mat facing out. Immediately thereafter an8-mil thick corrugated shield approximately 6.75 inch wide is wrappedover the corewrap. Both the corewrap laminate and the aluminum shieldoverlap approximately one-fourth inch. This assembly is passed through acrosshead extrusion die where an overall jacket of polyethylene some 60mils thick is coated onto the cable. The polyethylene is usually a highdensity or cross-linkable low density type with a high carbon contentfor aging stability. Extrusion temperatures are commonly in the 430475F. range, line speeds as low as 25 feet/min. The complete jacketed cableis then rapidly quenched in a water-bath and wound on a reel in 3000 ft.lengths ready for use as underground telephone cables and the like.

EXAMPLE 2 Laminate Preparation-A commercially available nonwovenpolyester mat such as DuPonts Spunbound Reemay 2017 (1.5 oz./sq. yd. orlbs/ream basis weight) is chosen as the ply which is highly bulked and apoor thermal conductor.

It is laminated on a conventional adhesives laminator to a 5- milpolypropylene film. The polypropylene is coated with apressure-sensitive adhesive, such as a rubber cement, with agravure-type applicator and the organic solvent is evaporated from thecoating in a forced convection oven. The adhesive coated polypropyleneis joined to the nonwoven polyester between two heated rolls forming thelaminating nip. Typical conditions are:

Adhesive Coating Weight 3 lbs/ream Drying Oven Temperature I50 F.Lamination Nip Temp. l50 F. Lamination Nip Pressure 30 lbs.lin. widthLamination Speed 250 feet/min.

The laminate is wound to a large diameter roll and slit subsequently tonarrow widths as with the laminate in example l.

Cable Manufacture-The cable is similar to that in example 1.

In order to illustrate the effectiveness of the laminated product ofexamples 1 and 2 for cable wrap purposes, see the comparative thermalconductivity values with prior art cable wraps by theGuarded Hot PlateMethod(ASTM C-l 77-63):

Thermal Conductivity BTU/Hr./Ft./F.

3-mil thick MYLAR film 345 IO-mil thick polypropylene film 92 Laminateof S-mil PP film with 30 lb./ream of polyester mat. 20

This large decrease in thermal conductivity then clearly illustrates thetremendous thermal conductivity decrease obtainable by the use of thelaminate of examples 1 and 2.

EXAMPLE 3 Preparation of the Laminate-A commercially availablecorrugated polyester film, such as DuPonts 2-mil corrugated Mylar, ischosen as the highly bulked ply with low thermal conductivity. It islaminated on a conventional adhesives laminator to a 2-mil polypropylenefilm. The polypropylene is coated with a pressure-sensitive adhesive,such as a rubber cement, with a gravure-type applicator and the organicsolvent is evaporated from the coating in a forced convection oven. Theadhesive coated polypropylene is joined to the corrugated Mylar betweentwo heated rolls forming the laminating nip. Typical conditions are:

Adhesive Coating Weight 3 lbs/ream Drying Oven Temperature F. LaminationNip Temp. 150 F. Lamination Nip Pressure 30 lbs./in. width LaminationSpeed 250 feet/min.

The laminate is wound to a large diameter roll and slit sub sequently tonarrow widths as with laminate in example 1. For' this example thecorewraps final width is approximately 4.25 inches.

Cable Manufacture-The cable is similar to that in example l, exceptsmaller in diameter. The core has 50 pairs of 19- gauge wire for a corediameter of approximately 0.66 inches. The 4.25-inch corewrap againpermits a one-fourth inch overlap. Extrusion jacketing speeds aretypically in the l25-l50 feet/min. range.

Example 4 Laminate Preparati0nln this example, a polyolefin foam is thehighly bulked ply. The laminate is formed by multiple extrusion, thatis, two extruders which simultaneously feed a single die. The design ofthe die (e.g. US. Pat. No. 3,223,761, U.S. Pat. No. 3,266,093) producesan extrudate with a core of one material (10 mils thickness ofpolypropylene foam) and an outer layer on both sides of the foam of asecond material (1 mil thickness of polyphenyleneoxide on each side).

Cable Manufacture-Similar to example 1.

Although this invention has been described with respect to only a fewexamples and embodiments, it is not to be so limited and changes andmodifications may be made therein which are fully within the intendedscope of the invention as 2. An electrical cable according to claim 1wherein said dielectric material is polypropylene having a thickness of5 mils, said porous nonwoven material is a polyester mat weighing 1.5ounces per square yard and said polyolefin insulating cover ispolyethylene having a thickness of about 70 mils.

1. An electrical cable comprising: a. multiple strands of conductors,said conductors individually insulated electrically from one another; b.said multiple strands wrapped with a composite laminate comprising afilm of flexible dielectric material, said film having an inner surfacecontiguous wIth said multiple strands, and an outer surface; c. saidouter surface of said film bonded to a porous nonwoven material having amaximum bulk density of one-half of the material from which it isformed; d. said dielectric material having a maximum dielectric constantof 3.5 measured at room temperature and 1 kc., and a dissipation factorin the range of 0.02 to 1.5 percent; e. a corrugated aluminum shielddisposed circumferentially about said composite laminate; and f. apolyolefin insulating cover disposed externally of said aluminum shield.2. An electrical cable according to claim 1 wherein said dielectricmaterial is polypropylene having a thickness of 5 mils, said porousnonwoven material is a polyester mat weighing 1.5 ounces per square yardand said polyolefin insulating cover is polyethylene having a thicknessof about 70 mils.